Chromate ion CrO

The chromates of the alkali metals and of magnesium and calcium are soluble in water the other chromates are insoluble. The chromate ion is yellow, but some insoluble chromates are red (for example silver chromate, Ag2Cr04). Chromates are often isomorph-ous with sulphates, which suggests that the chromate ion, CrO has a tetrahedral structure similar to that of the sulphate ion, SO4 Chromates may be prepared by oxidising chromium(III) salts the oxidation can be carried out by fusion with sodium peroxide, or by adding sodium peroxide to a solution of the chromium(IIl) salt. The use of sodium peroxide ensures an alkaline solution otherwise, under acid conditions, the chromate ion is converted into the orange-coloured dichromate ion ... 

In 1 M HC1, the formal potential is just 1.00 V and, in 2 M H2S04. it is 1.11 V so dichromate is a less powerful oxidizing agent than Mn04 or Ce4+. In basic solution, Cr20 is converted into yellow chromate ion (CrO -), whose oxidizing power is nil ... 

Acid-base equilibria of Cr(III) were summarized in Problem 10-35. Cr(VI) in aqueous solution above pH 6 exists as the yellow tetrahedral chromate ion, CrO . Between pH 2 and 6, Cr(VI) exists as an equilibrium mixture of HCr04 and orange-red dichromate, Cr30. Cr(VI) is a carcinogen, but Cr(III) is not considered to be as harmful. The following procedure was used to measure Cr(VI) in airborne particulate matter in workplaces. 

The chromate ion CrO has the shape of a tetrahedron. Deduce the symmetries of the normal modes and explain which of these are infra-red active and which are Raman active. 

Chromate and Dichromate Ions. In basic solutions above pH 6, Cr03 forms the tetrahedral yellow chromate ion CrO between pH 2 and 6, HCr04 and the orange-red dichromate ion Cr2Or are in equilibrium and at pH values <1 the main species is H2Cr04. The equilibria are the following ... 

Chromium(VI) species are oxidizing agents. Basic solutions containing chromate ions, CrO ", are weakly oxidizing. Acidification produces the dichromate ion, and... 

Anions such as iodide ions, I-, sulfide ions, S2-, and thiocyanate ions, SCN, are the soft base. In contrast, hydroxide ions, OH-, fluoride ions, F-, chloride ions, Cl-, phosphate ions, PO4-, sulfate ions, SO4-, and chromate ions, CrO -, are the hard base. Bromide ions, Br-, and sulfite ions, SO2-, are situated somewhere between the soft and the hard base. 

It is interesting to see that chromate ions, CrO, which are strongly hard in the Lewis base and an oxidizing agent, oxidize the iron atoms or ferrous ions on the metallic iron surface and reduce the chromate ions themselves forming a passive film of chromic-ferric oxide, which is extremely thin and highly corrosion resistive ... 

The totally symmetric breathing mode is also prominent in the resonance Raman spectra of the chromate ion CrO (Kiefer and Bernstein, 1972). In addition, this ion shows activity of degenerate modes, probably due to Jahn-Teller coupling (Tsuboi et al., 1975). Since most of the data pertain to the preresonance region, a complete analysis is impossible. However, we shall show that it is possible to account semiquantitatively for the observed REPs of the breathing mode (Siebrand and Zgierski, 1979b). 

Calculated and Observed Relative Scattering Cross Section of the Chromate Ion CrO " in... 

Aqueous chromate ion, CrO , exists in equilibrium with aqueous dichromate ion, Cr207, in an acidic solution. What effect will (a) increasing the dichromate ion and (b) adding HCl have on the equilibrium ... 

Polyatomic Anions from Transition Elements Chromium and manganese form some polyatomic anions that theoretically may be traced to acids, but only the ions are important. Their names and formulas are the chromate ion, CrO/ , the dichromate ion, Cr207 , and the permanganate ion, Mn04. ... 

Other reactions. Chromate ion, CrO/ precipitates yellow cadmium chromate only from concentrated solutions of Cd. ... 

Chromate ion, CrO/, is added to the mixture to indicate the end point. When excess Ag is added beyond the end point, red solid Ag2Cr04—which is more soluble than the white AgCl preeipitate— begins to form, indicating that the end point has been reached. 

Chromium in the +6 state forms two different oxoanions, the yellow chromate ion, CrO/ , and the red dichromate ion, Cr207 (Figure 20.11, page 628). The chromate ion is stable in basic or neutral solution in acid, it is converted to the dichromate ion ... 

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